It is, at times, advantageous to physically separate the particulate matter in a sample from the supernatant in which it is suspended. To effect this purpose it is common practice to utilize a device known as a "pelleting" centrifuge. The rotating member of such a device is, accordingly, termed a "pelleting" rotor. Most pelleting rotors are fixed angle rotors, known as such for the provision of cavities about the periphery thereof which are inclined at a predetermined fixed angle with respect to the vertical central axis of the rotor. The fixed angle may be zero degrees, defining a so-called "vertical tube" or "vertical angle" rotor. A fixed angle rotor is to be contrasted with a swinging bucket rotor in which sample carriers, or buckets, are pivotally mounted to the rotor and swing outwardly from a vertical to a horizontal position as the speed of rotation increases.
Rotor speed also serves as a mode of classification of centrifuge rotors. Rotors operable at speeds below approximately twenty thousand revolutions per minute are classified as "superspeed" rotors, while rotors which can operate above approximately twenty thousand revolutions per minute are called "ultraspeed" rotors.
For some time ultraspeed rotor users have utilized as a measure of performance of the rotor a factor known as the "clearing factor" K. It is well known that the clearing factor K associated with swinging bucket rotors is defined by the relationship (with dimensional constants omitted): ##EQU1## where, w is the angular velocity of a bucket about a reference axis CL, R.sub.max is the distance between the reference axis CL and the radially outermost boundary of the bucket therefrom, and
R.sub.min is the distance between the reference axis CL and the radially innermost point at which the sample bucket is located. The clearing factor K serves as an indication of the time required to pellet particles of a sample using a given rotor. The lower the K factor the shorter is the time required for a particle to pellet. However, this factor is usually of little use to the users of other centrifuges, such as superspeed centrifuges, since such users are typically concerned with larger numbers of particles and greater sample volumes. For example, users of ultraspeed centrifuges often do not utilize all of the rotor compartments due to limited sample quantity, whereas superspeed centrifuge users often require multiple runs to process a sample. Thus, when an ultraspeed centrifuge user is confronted by the problem of processing in a given time what to him is a large sample of supernatant and suspended particles, such a user is likely to respond by choosing the ultraspeed centrifuge rotor having a low K factor. Conversely, however, a user of a superspeed centrifuge when confronted by the problem of processing in a particular time what to him is a large sample of supernatant and suspended particles, would likely ignore the K factor as being inapplicable and most likely select the largest volume rotor and simply assume that this sized rotor provides the shortest processing time.
However, such reasoning, straightforward as it may seem, tends to overlook several drawbacks which attend large volume superspeed rotors and which, in fact, may make the largest sized rotor not as efficient in processing large quantities of a sample in a given time. For example, a larger volume rotor would tend to exhibit a concommitantly large physical configuration, which would lead to increased windage factors, thus detracting from rotor speed and the centrifugal force generated.
Accordingly, it is believed advantageous to provide a rotor in which the pelleting capacity of the rotor is based on considerations other than mere physical size or K factor.